Problem: Solve for $a$ : $15 = a + 8$
Explanation: Subtract $8$ from both sides: $ \begin{eqnarray} \\ \hphantom{15 {- 8}} &\hphantom{=}& \hphantom{a} \\ 15 &=& a + 8 \\ \\ {-8} && {-8} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 15 {- 8} &=& a \\ \end{eqnarray} $ Simplify. $ a = 7$